1. Technical Field
The present invention relates generally to transistor amplifiers and, more particularly, to a variable gain transistor amplifier which is capable of efficiently achieving high IP3 performance over a wide dynamic range.
2. Discussion
Modern commercial receiver applications such as those associated with global positioning satellites (GPS), high definition television (HDTV), cellular phone systems, and wireless local area network (LAN) systems increasingly require a wide input dynamic range as well as good intermodulation spur suppression. Systems such as these commonly operate within a frequency range of zero to 5.8 Gigahertz and require a large dynamic range due to varying signal strengths received from sources of various distances and locations. In addition, these systems also encompass high channel densities which require low intermodulation distortion.
In view of these and other requirements, there is an increasing need for high IP3 performance variable gain amplifiers which may adequately serve the needs of modern and future receiver applications. In the past, there have existed a number of conventional amplifiers that are capable of providing variable gain amplification. Generally, conventional variable gain amplifiers are typically constructed from a number of discrete transistors and PIN diodes. Typical variable gain amplifier circuits have included the use of bipolar transistor amplifiers such as a common-emitter transistor or a pair of Darlington-connected transistors with an input, output and feedback path. The feedback path usually receives a gain control signal for variably adjusting the amplification gain of the amplifier.
Variable gain amplifiers of the conventional type are capable of achieving variable gain amplification, but many of these approaches generally trade off performance and efficiency to achieve variable gain over a widened range. The conventional approaches usually fail to provide optimal compensation to the amplifier in an attempt to achieve enhanced IP3 performance, especially at high gain operations. As a consequence, the conventional amplifiers generally are not capable of efficiently achieving high IP3 performance while at the same time meeting the needs of a large input dynamic range.
It is therefore desirable to provide for a variable gain amplifier which may achieve high IP3 performance while maintaining a large dynamic gain control range. It is further desirable to provide for such a variable gain amplifier which provides for increased IP3 performance and gain while incurring a small increase in power dissipation. More specifically, it is desirable to provide for a variable gain amplifier with a PIN diode bias compensation network which enhances the amplifier performance, especially for higher gain states. Also, it is desirable to provide for an inexpensive self-biased variable gain amplifier circuit which employs heterojunction bipolar transistors on a monolithic chip.